Archive for the 'Hawkeye 50th Anniversary' Tag

This Thursday, 21 October 2010, marks the 50th anniversary of the first flight of the first purpose built AEW aircraft, the E-2 Hawkeye (actually, it was the YW2F-1). Designed around the radar, rather than adapting an existing airframe, the Hawkeye symbolized function over form – from the 24ft “rotodome” prominently perched over the fuselage, to the quadruple tail and twin turboprops. It wasn’t pretty – but then, it wasn’t meant to win beauty contests.

It was meant for far more deadly competition.

Read the rest of this entry »

1050L 24 Oct 1944. USS St. Lo (CVE-63) is under heavy air attack. After successfully fending off the superior surface force of VADM Takeo Kurita’s Center Force, “Taffy 3” is now defending against a surprise air attack that has lasted some 40 minutes already. One of the features of this attack is the use of suicide attacks.
The “Divine Wind” — Kamikazes.
In the midst of battle, St Lo is struck by a plane flown by Lt Yukio Seki. Penetrating the escort carrier’s unarmored flight deck, the plane and its bomb explode in the port hangar bay, igniting a massive fire with secondary explosions. When the bomb and torpedo magazine detonates, St. Lo is engulfed in flames and sinks 30 minutes later. Barely 6 days later, the carriers Franklin and Belleau Wood were struck by suicide aircraft. Both were forced to retire for repair before rejoining the fleet. This emerging threat, kamikaze attacks, were a hint of what was to come as the Fleet closed on the Japanese homeland. The urgency for getting Cadillac’s capabilities operationally deployed was being underscored by increasing losses in the Pacific…

Development & Production

AN/APS-20 Installation in AD3W (similar to earlier TBM-3W installation)

Recognizing the importance of the Cadillac system, an early decision was made by the Navy to establish production coincident with its development. To be sure, this imparted significant risk to the program, but in light of its benefits this was deemed acceptable. To facilitate this plan, the project was divided into five parts: shipboard system; airborne system; airborne radar; radar transmitter; and beacons and IFF. So far, what had been brought together was still not much more than a conceptual model – it was time for building actual sets. Development was undertaken in earnest shortly after approval in May 1944. Using ground-based radar located atop Mt. Cadillac and operating at low power to simulate the APS-20, work on the airborne elements, particularly the relay equipment was well underway. This arrangement allowed prolonged simulation of the air- and ship-board environment, contributing significantly to the shortened development timeline.

Progress was measured in the completion of each of the first 5 developmental sets envisioned. The first set flew in August 1944 –

AEW Radar Picture of Cape Cod from 20K ft.

barely 3 months after the approval to begin work was received. Each subsequent system saw incremental improvements over its predecessor with the improvements folded back into the earlier models. By October 1944 a full-fledged demonstration was flown for the benefit of USAAF and USN leaders. These demonstrations consisted of 2 aircraft and 1 shipboard set and were flown out of Bedford Airport (later known as Hanscom AFB), Massachusetts. By all accounts, the demonstration was extremely successful, which boded well for the production units, forty of which had been ordered by the Navy in July 1944.

AN/APS-20 Antenna installation on TBM-3W

As additional developmental sets were completed, permanent sites were established in Bedford and MIT (originally scheduled for Brigantine, NJ). The latter was established at MIT for the purpose of evaluating the system in the heavy interference conditions expected in the operational environment. It was in this environment that the first major problem was uncovered as the system was found to jam itself – interference was so bad that rotational data as transmitted by the double-pulsed coding and passed over the relay link was virtually completely jammed. An extraordinary effort though on the part of the development team led to a triple pulse encoding scheme. With little time to fully test this new set-up (there was considerable rework in the synchronizers, relay receivers and decoders to be accomplished), the third set was packed off to formal Navy trials at the CIC Group Training Center, Brigantine, NJ that started in January 1945 – only two weeks behind schedule

In December, at the height of the crisis over finding a means to address the interference problem, DCNO(Air) disclosed to Cadillac team leaders the urgency by which their equipment was required to combat the rapidly growing kamikaze threat. Even though Cadillac was already at the top of the Navy’s electronics development requirements, with the increased need, the Navy made available substantial numbers of officers, technicians, draftsmen and even a special air transport system to facilitate delivery of parts and personnel.

On the production side, a flexible system of generalized target dates were crystallized as designs firmed up, permitting incorporation of changes as experience was gained with the development units. Though this was undoubtedly the least economic process in terms of cost, the brute force development/production method was necessary to ensure delivery of the critical sets in time for the invasion of Japan — anything less than the very high priority Cadillac carried would have hampered successful completion. Nevertheless, a production schedule was agreed to in June with BuAer that would start deliveries of operational systems with two in February 1945. This was subsequently modified in November for initial delivery of 1 set in March 1945 followed by 4 in April and then 8 per month afterwards.

Operational Testing

Not long after starting operational evaluations at Brigantine, more problems were discovered, centered primarily on interference issues in the shipboard environment. Again, most of us today are well aware of the hazards the witches’ brew of RF in the CV environment. Mixtures of high-powered radars operating at different frequencies overlaid with HF, VHF and UHF voice comms provide an extremely challenging environment to develop and deploy a new system, even with the benefit of fifty plus years of experience. Without the benefit of that experience, the roadblocks encountered are not surprising. More modifications were made to the shipboard system with filters to screen out the extraneous radiation. Additionally, as more experience was gained with the APS-20 radar, it was determined that anti-clutter filters were needed to reduce the effect of large clutter discretes from the sea’s surface in and around the immediate vicinity AEW platform (typically out to 20 nm from ownship). Mounting the antenna above the airframe would have resolved this problem, using the aircraft itself to screen out large clutter discretes encountered from returns within 10-15 nm from the platform, but that was not an option for the Avenger platform.

On the West coast, training in the TBM-3W for pilots and crewmen was undertaken by the Fleet Airborne Electronics Training Unit (FAETU) in preparation for deployment. While the crews were in training, the USS Ranger (CV-4), recently returned from delivering aircraft to allied forces in Casablanca, entered Norfolk Naval Shipyard 17 May 1945 for a six-week overhaul, during which a CIC and the Cadillac shipboard equipment were installed. Underway again in July, she arrived at North Island on July 25th where she loaded aboard her airwing. This airwing was different from the conventional wing in that it included several developmental concepts; among these were the Cadillac configured TBM-3Ws and the Night Air Combat Training Unit from Barber’s Point. By August 1945 she was in Hawaiian waters conducting final CQ prior to leaving for Japanese waters when the war ended.

With the end of the war, Cadillac was almost, but not quite completed. While the carrier-based component did not have a chance to prove itself in combat, the utility of carrier-based AEW was so clear and its applications so far ranging in impact that further development and deployment would continue post-war, with deployments on Enterprise and Bunker Hill. In addition to the carrier-based component, a second development was begun under Cadillac II for a more robust airborne capability. That will be the subject for the next installment.

TBM-3W Data
Wing span: 54.2 ft
Length: 41.0 ft
Weight (empty): 11,893 lbs
Weight (max): 14,798 lbs
Max Speed: 260 mph @ 16,450 ft
Cruise: 144 mph
Svc ceiling: 28,500 ft
Range (scout): 845 miles

To Be Continued…

Project Cadillac (Part II)

 Part I 


Project Cadillac was more than just a program to develop radar – it would develop an entire AEW system — Radar, IFF, relay equipment, shipboard receivers, and airborne platform. Such an undertaking would be ambitious enough in peacetime, at the height of a critical stage in the war it bordered on a divine miracle. – SJS 



February 1944. In Europe the invasion of Italy is well underway and the Battle of Monte Casino is engaged. Eisenhower establishes SHAFE headquarters in Britain. The RAF drops 2300 tons on Berlin, the 8th AF begins the “Big Week” bombing campaign and Soviet troops continue the offensive begin at Novgorod and Leningrad. In the Pacific US forces have landed and captured the Marshall Islands and have moved on to Eniwetok Atoll. In the south, MacArthur’s forces have begun Operation Brewer in the Admiralty Islands. The tide, ever so imperceptibly, is turning in favor of the Allies. In Japan, Commander Asaiki Tamai asked a group of 23 talented student pilots, whom he had personally trained, to volunteer for a special attack force. All of the pilots raised both of their hands, thereby volunteering to join the operation.

MIT Radiation Lab (Feb 1944)

In the US, the fruits of scientific research and technological prowess were starting to manifest – high altitude bombers, Essex-class carriers, jet engines, the beginnings of nuclear weapons. At the MIT-RL, proposals were forwarded for an ambitious program to develop an AEW system that would be deployed with the fast carrier forces in the Pacific. It was envisioned that the system would be in place for Operation DOWNFALL, the projected invasion of the Japanese homeland, slated for sometime in early 1946. Following a series of meetings with reps from the Navy’s Bureau of Ordnance (BuOrd) the Navy formally requested the National Defense Research Committee (NDRC) to establish the project. Ultimately, the project would include 9 of MIT-RL’s 11 laboratories, BuAer, BuShips, Naval Air Modification Center, Philadelphia, Naval Research Lab, several Navy contractors and Radiation Lab subcontractors and over 160 officers and men. The project was eventually given the code name of CADILLAC, the name of the highest mountain on the US eastern seaboard and in the fall and winter, the location of the first sunrise in the lower 48 states. It would serve as the site of some of the developmental relay work because of its height and proximity to the sea.

As originally envisioned, Cadillac would consist of two sections (see CONOPS Illustration): one airborne (“AEW Aircraft”) and the other shipboard (“CV CIC”). The airborne unit would carry the APS-20 radar, IFF and VHF comms and relay equipment, acting as an airborne radar and relay platform for the ship. Back on the ship, the radar picture from the airborne unit would be relayed via a VHF video data link and displayed on a dedicated PPI (Plan Position Indicator) scope. Communications with far-flung fighter CAP would also be relayed through the airborne unit. Sorting out friend from foe would be via the newly developed IFF or Identification Friend Foe system which relied on an aircraft responding to electronic “challenge” signals with a coded pulse train. The airborne unit would also have the ability to display ownship’s radar picture and have a limited capability to control fighters, but this was planned to be a fall-back capability. 


Aircraft. The aircraft chosen was the only carrier-based aircraft large enough to accommodate the 8-foot radome and 2,300 lbs of associated equipment. Stripped of turret, armor, and armament, a TBM-3 Avenger served as the initial platform for Cadillac. Besides the Cadillac equipment, the XTBM-3W was modified to include an engine driven high power generator, additional tail stabilizers, addition of a crewman position in the aft fuselage and over 9 separate antennas on the fuselage, tail, and wings. 

Airborne System. The AN/APS-20, developed as part of the Cadillac program, was a 10cm set that had a peak power output of 1 megawatt and a 2-second pulse. The design of the APS-20 radar was so sound that variations of this same radar would see use well into the 1960s on a variety of USN, USAF and allied AEW platforms, until it was ultimately replaced by the E-2’s APS-96/120 series among others. The IFF system was built around the AN/APX-13 with a very high power (2 kW) transmitter and one of the most sensitive receivers in this type application. It was designed to enable ID of targets on both the (then) Navy standard A and G bands at ranges comparable to the radar. To “pipe” this information back to the ship, the AN/ART-22 relay-radar transmitter, broadcast the picture back to the ship on a 300 mc frequency.The radar synchronizer also synchronized the IFF and relay signals, encoding their outputs to ensure reception even in an environment characterizedby heavy enemy jamming and intrusion. Remote operation of the airborne system from the ship was made possible by the AN/ARW-35 receiver, AN/ARC-18 shipboard relay and the use of a modified flux gate valve to stabilize and orient the radar display to true North (ed. note – not altogether different from the system that was used in the E-2 almost 2 decades later). All this, of course, was in addition to the usual compliment of voice comm., IFF, and flight/navigation gear. Space, as one can see from the cutaway, was at a premium, even in the large-bodied Avenger. 

 Shipboard System. The shipboard system primarily consisted of relay (which included omnidirectional or a horizontal diversity receiver), decoding, and shipboard signal distribution equipment. The signal was passed to 2-3 PPI scopes, located in CIC. In CIC, the picture was merged with that of the ship in a manner that eliminated motion induced by the AEW platform – in other words, a ground-stabilized picture oriented to true north. That picture could be expanded to a 20nm view for detailed examination of sectors of interest. When tied together with voice communications, the implications of this capability were astounding. 

 Let us step back for a moment and review what the CONOP and “to be” Cadillac system would provide. Expanded radar coverage, in theory out to 200 nm. Positive identification of friendly aircraft in that volume of surveyed airspace. The ability to effect positive control of interceptors well closer to expected enemy marshaling points. Detect and track friendly and hostile surface units (including snorkeling submarines). Finally, the ability to bring all this information together and display it in CIC enabling informed decision-making from unit up to Fleet level. We who have been fortunate enough to have operated in the age of modern AEW aircraft, digital data links and automated detection and display systems take these for granted. It is not until one or more elements are removed that their intrinsic value is appreciated. This was something the Royal Navy painfully re-discovered during the war to reclaim the Falklands/Malvinas. That the concept, much less the hardware and integration of these many disparate elements was conceived and executed in a wartime situation says much about the technical verve and capabilities of this band of naval and civilian scientists, engineers and operators. The process of how this was brought to reality and IOC will be the subject of the next installment. 

To Be Continued

(crossposted at

Project CADILLAC (Part I)

Ed note: Everything has a beginning and that beginning is usually quite humble compared to present conditions. Consider, a small spring at the headwaters of the Madison River in Montana is the source of the mighty Missouri River which itself empties into ol’ man river — the Mississippi, all of which drain the better part of the country described in the Louisiana Purchase. Likewise, current day Airborne Early Warning and battle management, as we know it, sprang from humble beginnings and the collaborative efforts of the private and public sectors and borne in the urgency of war. Herewith then, the story of that effort is told as we begin the observance of the Hawkeye’s 50th Anniversary. – SJS

There is an arrogance permeating our culture such that it is widely believed that the (fill in the blank with the latest technological wonder) is (1) fairly recent in invention and (2) anything that preceded was hopelessly crude and unsophisticated, if it even existed or could have been possibly conceived in an earlier age. Serious students of history, particularly technological history, will assert though, the degree of inventiveness and technical complexity evidenced by our predecessors is indeed extraordinary, especially when put in context of the extent of knowledge in a particular field at the time. The story of airborne radar, and airborne early warning radar in particular, is one of the signatory lessons in this vein.

Radar was not unknown in the early days of WWII – indeed the story of how the CHAIN HOME radar stations, linked to coordination centers who in turn guided and directed Leigh-Mallory’s “big wing” fighter tactics is well known. The US Navy was already working to incorporate radar into its surface ships to permit gunnery under all weather/day-night conditions and meet navigational needs. Radar “expanded the battle space” (in the current parlance) but soon encountered problems – not the least of which was the curvature of the earth and the haven it provided to low flying aircraft. The solution, raise the radar antenna by mounting the radar to an aircraft, was fraught with a number of challenges.

Chief among those hurdles was the radar wave itself. The early search radars were low frequency (HF-band) with a long PRF (pulse repetition frequency) which provided the necessary range and were generally easy to generate. The down side was the requirement for large, very large antennas. Even later radars with parabolic antennas and operating at higher frequencies still tended to be very large. Airborne radar would need to be a microwave radar that provided high power with a smaller antenna. Simple in thought, difficult in execution. Yet efforts were underway on both sides of the Atlantic to meet this problem. The solution would be a device called a magnetron – specifically, a cavity magnetron.

Simple two-pole magnetrons were developed in the 1920s by Albert Hull at General Electric’s Research Laboratories (Schenectady, New York), as an outgrowth of his work on the magnetic control of vacuum tubes in an attempt to work around the patents held by Lee DeForest on electrostatic control. The two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency. The cavity version (properly referred to as a resonant-cavity magnetron), the path British scientists and engineers were working, proved to be far more useful.

In 1940, at the University of Birmingham in the UK, John Randall and Dr. Harry Boot produced a working prototype similar to Hollman’s cavity magnetron, but added liquid cooling and a stronger cavity. Randall and Boot soon managed to increase its power output 100-fold. Instead of giving up on the magnetron due to its frequency inaccuracy (in essence, what the Luftwaffe did), they instead sampled the output signal and synced their receiver to whatever frequency was actually being generated. An early 6kW version, built by GECRL (Wembley, UK) and given to the U.S. government in September 1940, was called “the most valuable cargo ever brought to our shores” (see Tizard Mission). At the time the most powerful equivalent microwave-producer available in the US (a klystron- basically a linear beam tube) had a power of only ten watts.

In the meantime, back in the US, work was underway on electronic relays as a means of extending the range of radar. The idea was to take multiple radars, deploy them at the limit of line-of-sight ranges and link those images into one centralized picture on the flagship. That line-of-sight range, of course, could be extended if the extended range platforms, or pickets, were airborne. As early as 14 Aug 1942, the MIT Radiation Lab (MIT-RL) demonstrated this capability using television equipment borrowed from RCA (actually with assistance from National Broadcasting Corporation (NBC) via a contract negotiated with RCA) and an experimental radar on the roof of another building. Further development and refinement led to the successful relay of radar signals to a receiver at East Boston Airport in May 1943 from an aircraft operating over Nantucket Island at 10,000 ft at a range of about 50 nm. In July 1943, the relay radar, the AN/APS-14 was demonstrated to naval officers at the East Boston Airport and a short film developed for COMINCH which was subsequently followed with a request to extend the range to 100 nm.

By the end of December 1943 even with the successful extension of range to 100 nm, however, there was no decision to proceed with production of the AN/APS-14 and there was movement to cancel the project. The following month though, the Navy proposed to develop an AEW system that had as part of the set-up, a high-power relay teamed with a high-power, microwave radar (enabled by the British magnetron). MIT-RL was awarded the task and Project CADILLAC was underway.

To Be Continued

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